One of the most confusing subjects is always: "What kind of sidescreens should there be on my MGA?" There are indeed a lot of different types, but there are only three basic types: the first is the screen with the hinged flap that came with the A1500 when it was introduced in 1955; the second was the screen with sliding windows introduced on the A1600 and the third was the aluminium screen that came with the hardtop. So far it's easy. Now the trouble of identifying these screens starts. The MGA 1500 had the screens with the hinged flaps, so you could have some fresh air or give signals to other drivers. They were available in the same colours as the hood, black or ice blue (parts catalogue MGA 1500: sidescreen with hinged flaps)
The flaps were kept closed by a spring, they didn't stay up by themselves. But very soon the aftermarket industry found just the solution for that particular problem, like this device made by Accessory Engineering (photo taken from the book "Accessories for your Sports Car" by Larry Reid, published by Sports Car Press, New York, 1958)
In fact these hinged screens aren't very handy; they give too much ventilation when you don't need it, too little when you do. So very soon screens with sliding windows became available on the aftermarket. They were often made of aluminium frames with acrylic plastic like the ones made by Stevens Plastics from Los Angeles or Lebron Manufacturing in Omaha, Nebraska
Stevens Lebron
In 1956 an aluminium hardtop was introduced as an option. It had much improved sidescreens, with aluminium frames and windows made of rigid plastic. The front window was fixed to the frame, but the rear window could slide in the frame. Strangely these screens were not available as standard on the 1500 (parts catalogue MGA 1500: side screen for use with aluminum hardtop)
They did become standard when the 1600 was introduced, although they were not of the aluminium type. Instead the metal frame was clothed in the same material as the hood. Again they were available in the same colours as the hood: grey, beige and blue (parts catalogue MGA 1600: sidescreen with cloth)
With the introduction of the A 1600 a glass-fibre version of the hardtop was introduced and that one also had aluminium screens with sliding windows, but with a slightly different curve of the screens (parts catalogue MGA 1600: side screen for use with glass-fibre hardtop)
New screens of all types are still being offered by companies like Anglo Parts or Moss (image taken from Anglo Parts Trim Catalogue 1990)